Concrete is effectively a sponge, water will pass through from a wet side to the dry side. So any moisture trapped under a vapor barrier will eventually pass back into the concrete when the ground dries out, unless the ground doesn't dry out in which case you'll eventually have a flood in your basement. This is why gutters and grading away from your house are important.
– BMitch♦Dec 17 '13 at 20:43

I think applying a penetrating sealer on the slab is a good idea. In an old TOH episode Tom Silva recommended putting roofing tar paper between the slab and 6 mil poly sheeting.
– OrganicLawnDIYDec 17 '13 at 21:07

So how would you install a wood floor over a concrete slab then? Would you use one of those underlayment products with air channels?
– iLikeDirtDec 17 '13 at 23:57

Tile mat taped & sealed but open to vent at the edges, plywood, then a non-vapor barrier underlayment, and the floating floor.
– BryceDec 18 '13 at 8:25

1

Also, what's the space between two moisture barriers? The 6 mil poly would be one... are you counting the foam underlayment as the second? What if the underlayment was made out of a material that didn't make it a moisture barrier, like cork?
– iLikeDirtDec 19 '13 at 14:44

Foam, especially with foil counts as a moisture barrier. Cork is far more permeable.
– BryceDec 19 '13 at 17:34

So how about just a moisture barrier type underlayment without the 6 mil poly?
– iLikeDirtDec 19 '13 at 21:22

I have used the panels that Home Depot have in stock as a base for a hardwood floor install with much success on to large floors. It ain't cheap, but it does the job. No need for nothing else, though the 6 Mil is a viable option, yes it should roll up the wall a little before drywall. That will be tough to protect, but it needs to vent to the dead space behind the wall.
Dricore is this product name, Subflor, with one "o" was another brand I used. I glued the T&G, although it says it did not need it. I wanted to insure it would not separate.

Test your floor for moisture, to be sure it is a dry as you think it is. Take a piece of poly about 1-2 ft. square and tape it securely to the floor with a strong tape. if it stays dry after a few days, it truly is dry, if it has condensate under it, do consider what I have above.
– JackDec 15 '13 at 14:28

I used these with no glue, no nails, no screws and no problems :)
– OrganicLawnDIYDec 17 '13 at 20:58

Taping down a piece of plastic does NOT work! The condensation you do or don't see comes from the ambient air temperature compared to the temperature of the concrete slab. It tells you nothing about the moisture content of the concrete. Why does a glass of ice water sweat when it's placed in a warm setting and a glass of room temperature water doesn't? The same glass of ice water will not sweat when placed in a cold environment "think refrigerator". This test only sets you up for failure.

Nah, no failure a year later. Everything's bone dry. And no, I did not "throw down a piece of plastic"; I used an approved foam underlayment product.
– iLikeDirtDec 2 '15 at 1:25

Since the concrete is porous, the correct metaphor is placing a wet sponge inside a plastic bag, and that does prevent the wood floor from getting wet. If your home is so humid and floor is so cold that it's condensing water, then the windows and doors will likely be covered long before the floor.
– BMitch♦Dec 2 '15 at 1:56